Method of producing a conductive silicon carbide-based sintered compact

ABSTRACT

A process is described for producing a conductive sintered body based on silicon carbide, in which 
     a) silicon carbide particles, optionally pretreated with a surface modifier, are dispersed in an aqueous and/or organic medium and positive or negative surface charges are generated on the silicon carbide particles by adjustment of the pH of the dispersion obtained; 
     b) carbon black and boron carbide are mixed in as sintering aids, where at least the carbon black particles have a surface charge opposite to the surface charge of the silicon carbide particles and the boron carbide can also be added, completely or in part, at a later point in time (stage c′)); 
     c) the slip thus obtained is shaped directly to form a green body or 
     c′) a sinterable powder is isolated from the slip obtained and is shaped to form a green body, where the above boron carbide can also be added to this sinterable powder; and 
     d) the green body obtained is subjected to pressureless sintering to form a sintered body in essentially three successive steps, namely (i) preheating to 1200-1900° C. in a nitrogen containing atmosphere (ii), sintering at 1900-2200° C. in a noble gas atmospher and (iii) post-heating at 2150-1850° C. and subsequent cooling to ambient temperature in an atomosphere containing nitrogen and/or carbon monoxide.

Materials based on silicon carbide have been known for some time and areutilized in a variety of ways for producing components. They have aseries of interesting properties, including the low density, the highhardness, the low coefficient of thermal expansion, the good oxidationand corrosion resistance and also a favourable creep behaviour and highthermal conductivity.

Furthermore, pure SiC has semiconducting properties, with thecorresponding electrical behaviour. Of particular interest here ispressureless sintered SiC, since it combines most of the abovementionedproperties with a very good thermomechanical behaviour (high strength athigh temperature).

It is known from the prior art that, only by using sintering aids, SiCcan be subjected to pressureless sintering. Possible sintering aidswhich have been described are a great number of different compounds andmaterial combinations, including, inter alia, metals such as aluminium,iron, lithium or magnesium and also metal oxides such as aluminiumoxide, beryllium oxide and rare earth metal oxides. However, onlycombinations of carbon/boron, carbon/boron carbide and carbon/aluminiumhave become established as sintering aids in industrial use. It is worthnoting that only small amounts of sintering aids are required to achievevirtually complete densification. The values published hitherto forcarbon are between 1.5 and 2.6% by weight and for boron or boron carbidebetween 0.3 and 1% by weight, based on silicon carbide used. Duringsintering, the carbon acts as a reducing agent and cleans the grainsurface of the SiC of SiO₂. Associated therewith is an increase in thesurface energy of the powder and the grain boundary diffusion duringsintering. In contrast, boron is incorporated at the grain boundariesand increases the volume diffusion during sintering. At the same time itacts against grain growth. To enable use to be made of theseadvantageous properties of the sintering aids, they have to bedistributed homogeneously in the green ceramic. The necessaryhomogeneity can be achieved in various ways. Frequently, the powdermixture comprising SiC and sintering aids is subjected to intensive wetmilling in the presence of surface-active substances. A particularlyhigh homogeneity is achieved when the individual SiC particles arecoated directly with nanosize sintering aids (e.g. nanosize carbonblack), as is disclosed in DE-A-42 33 626.

In addition, it is known that production of SiC materials having a goodelectrical conductivity requires dopants. These dopants include, interalia, aluminium nitride, molybdenum disilicide, phosphorus, arsenic andantimony. However, these additives have an unfavourable influence on thesintering behaviour of the ceramic, so that sufficient densification canonly be achieved by pressure-supported sintering processes (hotpressing, hot isostatic pressing), but SiC ceramics produced by thesemethods still have a relatively high porosity and have only limitedoxidation stability in air at high temperature.

Accordingly, it is an object of the present invention to produce SiCmaterials having good electrical properties, in particular goodelectrical conductivity, good oxidation resistance and high strength bypressureless sintering.

It has surprisingly been found that this object can be achieved by meansof the system (α-)SiC/B₄C/carbon if the green bodies are produced by theprocess described in DE-A-42 33 626 and the green bodies are subjectedto a multistage sintering process which is carried out at least in partin the presence of nitrogen.

The present invention accordingly provides a process for producing aconductive sintered body based on silicon carbide, in which

a) (preferably α-)silicon carbide particles, which may have beenpretreated with a surface modifier, are dispersed in an aqueous and/ororganic medium and positive or negative surface charges are generated onthe silicon carbide particles by adjustment of the pH of the dispersionobtained;

b) carbon black and boron carbide are mixed in as sintering aids, whereat least the carbon black particles have a surface charge opposite tothe surface charge of the silicon carbide particles and the boroncarbide can also be added, completely or in part, at a later point intime (stage c′));

c) the slip obtained after stage b) is shaped directly to form a greenbody or

c′) a sinterable powder is isolated from the slip obtained and is shapedto form a green body, where the above boron carbide (completely or inpart) can also be added to this sinterable powder; and

d) the green body obtained is subjected to pressureless sintering toform a sintered body,

where the process is characterized in that said stage d) is carried outin essentially three successive steps, namely (i) preheating to1200-1900° C., (ii) sintering at 1900-2200° C. and (iii) post-heating at2150-1850° C. and subsequent cooling to ambient temperature, and saidstep (i) is carried out in a nitrogen-containing atmosphere, said step(ii) is carried out in a noble gas (preferably argon) atmosphere andsaid step (iii) is carried out in an atmosphere containing nitrogenand/or carbon monoxide.

In a modification of this process, the slip obtained after stage (b) isapplied to a sintering-resistant substrate and dried and the substratethus coated is sintered as described in stage d).

As already mentioned above, the stages (a) to (c) of the process of theinvention are carried out as described in DE-A-42 33 626, which in termsof details is hereby expressly incorporated by reference.

In stage (a), the silicon carbide powder is suspended in water and/ororganic media.

Suitable organic dispersion media are especially water-miscible organicsolvents such as alcohols, esters, ketones, dimethylfomamide anddimethyl sulphoxide.

The Si—OH groups present on the surface of the SiC particles areconverted in the presence of protons or hydroxyl ions into chargedgroups Si—OH₂ ⁺ or Si—O⁻, which give rise to an electrostatic repulsionof the fine SiC particles and, thus, to a finely dispersed suspension.

Preferably, the formation of negative or positive surface charges iseffected or aided by addition of an acid or base. Suitable acids forthis purpose are, for is example, inorganic acids such as HCl, HNO₃,H₃PO₄, H₂SO₄ and also organic carboxylic acids such as acetic acid,propionic acid, citric acid, succinic acid, oxalic acid and benzoicacid. Suitable bases are, for example, NH₃, NaOH, KOH, Ca(OH)₂ and alsoprimary, secondary and tertiary, aliphatic and aromatic amines andtetraalkylammonium hydroxides. It is likewise possible to use acidic orbasic polyelectrolytes such as polyacrylic acid, polymethacrylic acid,polysulphonic acids, polycarboxylic acids and salts (e.g. having Na⁺ orNH₄ ⁺ as cations) of these compounds.

The surface charges generated in this way can be measured as the zetapotential. The zeta potential is pH-dependent and is either positive ornegative in relation to the isoelectric point of the respective material(e.g. the SiC). As a result of the electrostatic charging with the samepolarity, the dispersed individual particles remain stable insuspension.

According to a preferred embodiment of the present invention, the SiCpowder is subjected to a surface modification before formation of thesurface charges. According to the invention, this surface modificationis carried out by coating the SiC with a surface modifier havingfunctional groups which can be converted into negatively or positivelycharged groups by establishing an appropriate pH.

Suitable surface modifiers are, for example, silanes, acid chlorides,carboxamides, carboxylic anhydrides and carboxylic esters and alsoorganic polyelectrolytes such as polyacrylic acid, polymethacrylic acid,polysulphonic acids, polycarboxylic acids and salts thereof.

Examples of silanes which can be used aremercaptopropyltrimethoxysilane, 3-(trimethoxysilyl)propyl methacrylate,3-(triethoxysilyl)propylsuccinic anhydride, cyanoethyltrimethoxysilane,3-thiocyanatopropyltriethoxysilane,3-(2-aminoethylamino)propyltrimethoxysilane,3-aminopropyltriethoxysilane, 7-oct-1-enyltrimethoxysilane,phenyltrimethoxysilane, n-butyltrimethoxysilane,n-octyltrimethoxysilane, n-decyltrimethoxysilane,n-dodecyltriethoxysilane, n-hexadecyltrimethoxysilane,n-octadecyltrimethoxysilane, n-octadecyltrichlorosilane,dichloromethylvinylsilane, diethoxymethylvinylsilane,dimethyloctadecylmethoxysilane,tert-butyldimethylchlorosilylmethyldisilazane, diethoxydimethylsilane,diethyl trimethylsilylphosphite, 2-(diphenylmethylsilyl)ethanol,diphenylsilanediol, ethyl(diphenylmethylsilyl)acetate, ethyl2,2,5,5-tetramethyl-1,2,5-azadisilolidine-1-acetate,ethyltriethoxysilane, hydroxytriphenylsilane, trimethylethoxysilane,trimethylsilyl acetate, allyldimethylchlorosilane,(3-cyanopropyl)dimethylchlorosilane and vinyltriethoxysilane.

Examples of acid chlorides which can be used are acetyl chloride,propanoyl chloride, butanoyl chloride and valeryl chloride. Carboxylicanhydrides which can be used are, for example, acetic anhydride andpropionic anhydride. Suitable carboxylic esters are, for example, ethylacetate and a suitable carboxamide is acetamide.

To carry out the surface modification, the SiC particles are usuallysuspended in a nonpolar, aprotic solvent, e.g. an aliphatic or aromatichydrocarbon such as hexane or toluene or an ether such as diethyl etheror THF and admixed with the surface modifier.

The solvent can subsequently be stripped off and the surface-modifiedmaterial can be resuspended in an aqueous or organic medium, after whichpositive or negative surface charges are generated on the modifiedmaterial by establishing an appropriate pH. If the surface modifiercontains, for example, basic groups, as is the case for aminosilanes ormercaptosilanes, positive surface charges can be generated byestablishing an acidic pH. On the other hand, if the surface modifiercontains acidic groups, as is the case for carboxysilanes, negativesurface charges are generated by establishing a basic pH.

The (α-)SiC used in the process of the invention preferably has aparticle size of from 0.005 to 100 μm, in particular from 0.01 to 50 μm,particularly preferably from 0.05 to 5 μm, and can be either in the formof SiC powder or in the form of whiskers, platelets or fibres.

The particle size of the first sintering aid carbon (carbon black)depends on the fineness of the SiC particles; the carbon particlesshould be increasingly fine, the finer the SiC particles. In general,the carbon has a particle size of from 1 to 100 mn, preferably from 5 to80 nm, particularly preferably from 5 to 50 nm. The particle size of thesecond sintering aid B₄C is usually from 0.0001 to 10 μm, preferablyfrom 0.0005 to 5 μm, particularly preferably from 0.01 to 1 μm.

In the process of the invention, at least the sintering aid carbon blackis mixed into the SiC slip in a state in which the carbon particles havesurface charges of a polarity which is opposite to that of the surfacecharge on the SiC particles. This makes possible a uniform distributionof the sintering aid carbon black on the surface of the SiC particles.

An electrostatic charge on the sintering aid carbon black which isopposite to that on the SiC can be achieved, for example, by using typesof carbon black having acidic or basic surface groups. Basic types ofcarbon black are obtained, for example, in the furnace black process ina reducing atmosphere. Acidic types of carbon black are formed, forexample, in the gas black process in an oxidizing atmosphere. Commercialbasic carbon blacks are, for example, PRINTEX A, G, L, L6 and P, PRINTEX3, 25, 30, 40, 45, 55, 60, 75, 80, 85, 90, 95, 200 and 300 from DEGUSSA.Commercial acidic carbon blacks are, for example, Farbruβ FW 1, FW 2, FW2V, FW 18, FW 200, S 160, and S 170, Spezialschwarz 4, 4A, 5, 6, 100,250, 350 and 550, PRINTEX 150T, U, V, 140 U and 140 V from DEGUSSA.

On mixing the SiC suspension and the sintering aid (carbon black), thelatter adheres firmly to the surface of the SiC particles owing to thestrong electrostatic attraction. In the process of the invention, thecarbon black is always provided with a surface charge opposite to thatof the SiC, since otherwise stable deposition and uniform distributionon the surface of the SiC particles is not possible. The secondsintering aid component, i.e. B₄C, can, if desired, be mixed into theslip in uncharged form. If desired, the B₄C can be added only laterafter producing a sinterable powder and redispersion with the aid ofnonionic surfactants.

The amount of sintering aids added is usually, based on the SiC, from0.1 to 5% by weight, preferably from 0.5 to 3% by weight, of carbon(carbon black) and from 0.05 to 5% by weight, preferably from 0.1 to 3%by weight and in particular from 0.2 to 2% by weight, of B₄C.

The ceramic slip obtained after stage (b) of the process of theinvention, which generally has a solids content of from 10 to 60% byvolume, is further processed in a customary manner to give a green body.The slip can be shaped directly to form a green body by, for example,tape casting, slip casting, pressure casting, injection moulding,electrophoresis, extrusion, hot casting, gel casting, freeze casting,freeze injection moulding or centrifugation.

However, this slip can also be used to produce coatings by means ofcustomary wet coating techniques such as dipping, spraying, spin-coatingor doctor-blade coating.

Alternatively, a sinterable powder can be isolated from the slip, forexample by filtration, evaporation of the dispersion medium and spraydrying or freeze drying. The sinterable powder obtained is then eitherpressed as such to form a green body, or else the sinterable powder isredispersed, preferably using a surfactant as dispersant, and thesuspension is then processed according to one of the abovementionedshaping processes to form a green body. In this embodiment, suitabledispersants are, for example, inorganic acids such as HCl, HNO₃ andH₃PO₄; organic acids such as acetic acid, propionic acid, citric acidand succinic acid; inorganic bases such as NaOH, KOH and Ca(OH)₂;organic bases such as primary, secondary and tertiary amines and alsotetraalkylammonium hydroxides; organic polyelectrolytes such aspolyacrylic acid, polymethacrylic acid, polysulphonic acids,polycarboxylic acids, salts (e.g. Na or NH₄) of these compounds,N,N-dialkylimidazolines and N-alkylpyridinium salts; or nonionicsurfactants such as polyethylene oxide, fatty acid alkylolamides, fattyacid esters of sucrose, trialkylamine oxides and fatty acid esters ofpolyhydroxy compounds.

The green body or the layer is subsequently sintered to form a sinteredbody. While conventional sintering of the green body produces an SiCceramic which has many advantageous properties and as a result is usefulin many fields, as are indicated, for example, in DE-A-42 33 626, theSiC sintered body thus obtained has a low electrical conductivity andcan thus not be used in fields in which a good electrical conductivityis required, for example for the manufacture of electric igniters.

It has been found according to the invention that it is possible to givethe SiC sintered body, without impairing its other advantageousproperties, also a good electrical conductivity (specific resistanceusually from 0.5 to 10Ω×cm), if the sintering stage (d) is carried outunder specific conditions.

According to the invention, the sintering stage is essentially carriedout in three steps or phases, namely

(i) a preheating phase (if desired carried out in a plurality of stages)at a final temperature of 1200-1900° C., preferably 1500-1850° C., inparticular 1600-1800° C. and particularly preferably 1650-1750° C;

(ii) a sintering phase at a temperature of 1900-2200° C., preferably1950-2150° C. and particularly preferably 2000-2100° C.; and

(iii) a post-heating phase at a temperature not above and preferablybelow the sintering temperature, i.e. at a temperature of 2150-1850° C.,preferably 2100-1950° C. and particularly preferably 2100-2000° C.,followed by cooling the sintered body to ambient temperature.

Apart from the temperature, the gas atmosphere in which the individualsintering phases are carried out is a further important feature of thepresent invention. According to the invention, the gas atmosphere in theindividual phases has the following composition:

Phase (i): (non-oxidizing) nitrogen-containing atmosphere preferablycontaining at least 50, in particular at least 75 and particularlypreferably at least 90, per cent by volume of nitrogen, where theremainder is composed of one or more gases which are inert towardssilicon carbide (and silicon nitride) at the temperature used. Thesegases are preferably noble gases, in particular argon. Use is usuallymade of an atmosphere of 100% N₂.

Phase (ii): Noble gas atmosphere, in particular argon atmosphere. Otherinert gases (or nitrogen) can also be present in this atmosphere,although this is not preferred.

Phase (iii): (non-oxidizing) atmosphere containing nitrogen and/orcarbon monoxide. Besides nitrogen and/or carbon monoxide, it is alsopossible for other gases as can also be present in Phase (i) to bepresent. Although atmospheres of 100% of nitrogen or carbon monoxide canbe used, particularly preferred atmospheres are those in which nitrogenis present in admixture with CO and/or noble gas (in particular argon).In such mixtures, the nitrogen preferably makes up at least 75, inparticular at least 90 and particularly preferably at least 95, per centby volume of the mixture.

In the individual phases (i) to (iii), the green body or sintered bodyis first heated or cooled to the temperature indicated and then held atthis temperature for a certain period of time (known as hold time). Theoptimum hold time is a function of many factors, e.g. temperature,composition of the gas atmosphere, shape (in particular thickness) ofthe green or sintered body, construction of the heating apparatus, etc.The hold time is usually in the range from 5 minutes to 24 hours, morefrequently from 10 minutes to 12 hours.

The process of the invention enables the production by pressurelesssintering of an SiC ceramic which usually has the following properties:

density>85% of theory

specific resistance of from 0.5 to 10Ω×cm (e.g. able to be adjusted bymeans of the conditions in stage (d), see examples)

good oxidation resistance (because of closed, finely distributed pores)

fine-grained, uniform microstructure having a mean grain size of about 5μm

high strength (>300 MPa).

The sintered bodies which can be produced according to the invention areemployed in all areas in which electrical heating is used. Aparticularly preferred use is that in the form of an electric (glow)igniter. The resistance of such an igniter can be adjusted by means ofits geometry. Electric igniters produced from sintered bodies accordingto the invention can be operated, for example, at 220 volts and can bemade very small. However, the sintered bodies which can be producedaccording to the invention are also suitable for use as large-volumecomponents, e.g. a s SiC honeycombs which can be employed, for example,as diesel soot filters. In addition, the slip obtained in the process ofthe invention can be applied to (sintering-resistant) substrates andthen be further processed like a green body, which enables electricallyconductive layers on these substrates to be produced.

The following examples serve to illustrate the present invention withoutlimiting it.

PREPARATIVE EXAMPLE 1 Electrostatic Coating of SiC Powders With NanosizeCarbon

3 g of carbon black (PRINTEX 90 from DEGUSSA) are dispersed in 200 ml ofwater at pH 5-6 in a stirred ball mill. 150 g of SiC powder are added tothis suspension and the mixture is dispersed by milling for 2 hours. Thecontents of the mill are then allowed to settle and the liquid isfiltered via a filter press. The powder obtained is dried for 10 hoursat 90° C.

PREPARATIVE EXAMPLE 2 Preparation of an Aqueous SiC Slip FromElectrostatically Coated Powder

150 g of the SiC powder prepared in Preparative Example 1 and 0.97 g ofB₄C are, with addition of 2% by weight of a nonionic dispersant (TWEEN80 from ICI), dispersed in 70 ml of water in a stirred ball mill. Thesuspension obtained contains 40% by volume of SiC, 2% by weight ofcarbon black and 0.65% by weight of B₄C. The viscosity is <15 mPa s.

PREPARATIVE EXAMPLE 3 Production of Green Bodies

The suspension prepared according to Preparative Example 2 is, afterallowing the milling media to settle, used to produce a green body byslip casting and the green body is dried. The latter has a green densityof ≧60% of theory, a very narrow pore size distribution around 100 nmand is notable for a homogeneous carbon distribution over the entirethickness of the body (≦5 cm).

PREPARATIVE EXAMPLE 4 Surface-Modification of SiC Powder Using Silanes

The organoalkoxysilanes specified in the table below are dissolved in100 ml of toluene. 50 g of SiC powder are added to the solution whilestirring continuously. After addition is complete, the mixture is heatedunder reflux. After a reaction time of 5 hours, the hot suspension isfiltered and the filter cake is washed with toluene. The moist powder issubsequently dried for 12 hours at 115° C.

TABLE Silanes Amount (g) Mercaptopropyltrimethoxysilane 1.0473-(Trimethoxysilyl)propyl methacrylate 1.324 3-(Triethoxysilyl)propylsuccinic anhydride 1.621Cyanoethyltrimethoxysilane 0.933 3-Thiocyanatopropyltriethoxysilane1.179 3-(2-Aminoethylamino)propyltrimethoxysilane 1.184 3-Aminopropyltriethoxysilane 1.179 7-Oct-1-enyltrimethoxysilane 1.239Phenyltrimethoxysilane 1.057 n-Butyltrimethoxysilane 0.951n-Octyltrimethoxysilane 1.250 n-Decyltrimethoxysilane 1.397n-Dodecyltriethoxysilane 1.774 n-Hexadecyltrimethoxysilane 1.736n-Octadecyltrimethoxysilane 1.998 n-Octadecyltrichlorosilane 2.066Dichloromethylvinylsilane 0.751 Diethoxymethylvinylsilane 0.855Dimethyloctadecylmethoxysilane 1.828

EXAMPLES 1 TO 4

A green body having dimensions of 100×100×5 mm was produced by slipcasting or pressure slip casting (pressure in the range 5-50 bar) from aslip having the composition: 97.87% by weight of α-SiC having a specificsurface area of about 15 m²/g, 0.13% by weight of B₄C having a specificsurface area of about 15 m²/g and 2% by weight of carbon black having aspecific surface area of 250-300 m²/g, which had been electrostaticallydeposited on the SiC. The green body had a density of >55% of theory anda pore distribution in the range from 40 to 100 nm.

To produce an electrically conductive SiC ceramic, the followingsintering programme was employed:

(i) heating to 600° C. at 3 K/minute, hold time of 30 minutes, furtherheating to 1700° C. at 15 K/minute, hold time of from 60 to 120 minutes,in each case in a pure nitrogen atmosphere.

(ii) switching over to a pure argon atmosphere and then heating at 15K/minute to the temperature indicated below, with the hold timeindicated below at this temperature.

(iii) subsequent post-heating under the conditions indicated below andcooling to room temperature at 15 K/minute.

EXAMPLE 1 Sintering temperature: 2100° C Hold time: 30 minutesPost-heating phase: Temperature 2100° C. hold time 60 minutes;Atmosphere of N₂ containing 5 percent by volume of CO Specificresistance: 2 Ω × cm Sintered density: 2.87 g/cm³ Strength: >300 MPaMean pore size: about 1.5 μm. EXAMPLE 2 Sintering temperature: 2120° C.Hold time: 30 minutes Post-heating phase: Temperature 2100° C., holdtime 60 minutes; Atmosphere of N₂ containing 5 percent by volume of COSpecific resistance: 4.5 Ω × cm Sintered density: 2.90 g/cm³Strength: >350 MPa Mean pore size: about 2 μm. EXAMPLE 3 Sinteringtemperature: 2130° C. Hold time: 30 minutes Post-heating phase:Temperature 2100° C., hold time 60 minutes; Atmosphere of N₂ containing5 percent by volume of CO Specific resistance: 7 Ω × cm Sintereddensity: 2.93 g/cm³ Strength: >450 MPa Mean pore size: about 2 μm.EXAMPLE 4 Sintering temperature: 2125° C. Hold time: 30 minutesPost-heating phase: Temperature 2100° C., hold time 60 minutes;Atmosphere of N² containing 5 percent by volume of CO Specificresistance: 5.5 Ω × cm Sintered density: 2.92 g/cm³ Strength: >400 MPaMean pore size: about 2 μm.

APPLICATION EXAMPLE Production of an Electric Igniter

A dumbbell-shaped electric igniter having a total length of 60 mm(length of the two end pieces 20 mm), a width of 4 mm or 2 mm and athickness of 1 mm was produced mechanically from a plate produced asdescribed in Example 4 (specific resistance 5.5Ω×cm).

The igniter had contacts applied at its ends and was operated usingvoltages of from 100 to 160 V. The surface temperatures shown in FIG. 1resulted (R₀=1710Ω).

To determine the cyclic fatigue and ageing behaviour, the igniter wasoperated over 100,000 switching cycles. Even after the last switchingcycle, no changes in the material properties could be observed.

To determine the oxidation resistance, the electric igniter was heatedin air to 1300° C. and held at this temperature for 300 minutes. Therewas a parabolic increase in weight which became less with increasingdensity, with an almost constant value becoming established after fromabout 200 to 250 minutes (see FIG. 2).

What is claimed is:
 1. A process for producing a conductive sinteredbody based on silicon carbide, comprising: (a) dispersing siliconcarbide particles in an aqueous medium, an organic medium, or a mixturethereof, and generating postive or negative surface charges on thesilicon carbide particles by adjustment of the pH of the dispersion; (b)adding carbon black particles, and optionally adding boron carbideparticles, to the dispersion, where at least the carbon black particleshave a surface charge opposite to the surface charge of the siliconcarbide particles, to form a slip; (c) shaping the slip to form a greenbody, provided that this step (c) may be carried out only if boroncarbide particles were added in step (b), or (c′) (i) isolating asinterable powder from the slip, (ii) optionally adding boron carbideparticles to the sinterable powder, provided that boron carbideparticles are added in this step (c′) (ii) if they were not added instep (b), and (iii) shaping the sinterable powder to form a green body;and (d) pressureless sintering the green body in the three steps of (i)preheating to 120014 1900° C. in a nitrogen-containing atmosphere, (ii)sintering at 1900-2200° C. in a noble gas atmosphere, and (iii)post-heating at 1850-2150° C. and subsequent cooling to ambienttemperature in an atmosphere containing nitrogen or carbon monoxide or amixture thereof.
 2. A process according to claim 1 wherein, in step (d)(i), the atmosphere comprises nitrogen, optionally in admixture with notmore than 50% by volume of argon.
 3. A process according to claim 1wherein, in step (d) (iii), the atmosphere comprises nitrogen or carbonmonoxide or a mixture thereof, optionally in admixture with argon.
 4. Aprocess according to claim 1 wherein the atmosphere in step (d) (i)consists of pure nitrogen, and/or the atmosphere in step (d) (iii)consists of at least 50% by volume of nitrogen in admixture with carbonmonoxide or argon or a mixture thereof.
 5. A process according to claim1 wherein the temperature in step (d) (i) is 1500-1850° C., and/or thetemperature in step (d) (ii) is 1950-2150° C., and/or the temperature ofpost-heating in step (d) (iii) is 2000-2100° C.
 6. A process accordingto claim 1 wherein the hold times within the temperature rangesindicated in steps (d) (i) to (d) (iii) are from 5 minutes to 24 hoursin each step.
 7. A process according to claim 1 wherein the siliconcarbide particles of step (a) have been treated with a surface modifierhaving functional groups which can be converted into negatively orpositively charged groups at an appropriate pH.
 8. A process accordingto claim 1 wherein the silicon carbide particles of step (a) have beentreated with a surface modifier selected from the group consisting ofsilanes, acid chlorides, carboxamides, carboxylic anhydrides, carboxylicesters, and organic polyelectrolytes.
 9. A process according to claim 1wherein the silicon carbide is α-SiC.
 10. A process according to claim 1wherein step (c) comprises applying the slip to a sintering-resistantsubstrate and shaping it to form a green body on that substrate and step(d) comprises sintering the substrate with the green body thereon.